Answer
The third ionization energy requires removing an electron from a new p sub-orbital
Work Step by Step
Taking an electron from a full p-orbital will require a lot more energy due to a much larger increase in effective nuclear charge
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.